The four main types of blood are A, B, AB, and O. These groups are determined by the ABO system, which classifies blood based on specific molecules sitting on the surface of your red blood cells. Each type also comes in a positive or negative variety depending on a separate protein called the Rh factor, bringing the practical total to eight common blood types. But the core grouping always comes back to those four letters.
How the Four Blood Types Work
Your blood type depends on two things: the markers (called antigens) on your red blood cells and the antibodies floating in your plasma. These two features work as a lock-and-key system that your immune system uses to tell “self” from “foreign.”
- Type A has A antigens on red blood cells and carries antibodies against B.
- Type B has B antigens on red blood cells and carries antibodies against A.
- Type AB has both A and B antigens and carries no antibodies against either.
- Type O has neither A nor B antigens but carries antibodies against both.
This pattern, first described by Karl Landsteiner in 1901, follows a simple rule: your body never makes antibodies against its own markers, but it does produce antibodies against whichever markers it lacks. That’s why mixing incompatible blood types triggers a dangerous immune reaction. Your antibodies latch onto the foreign red blood cells and destroy them, a process called hemolysis. The debris from those destroyed cells can overwhelm the kidneys and cause organ failure.
What the Rh Factor Adds
The Rh factor is an inherited protein on the surface of red blood cells, completely separate from the A and B markers. If you have it, your blood type gets a “positive” label. If you don’t, it’s “negative.” So type A becomes either A+ or A-, type O becomes O+ or O-, and so on for all four groups.
Rh status matters most during pregnancy and transfusions. An Rh-negative person who receives Rh-positive blood can develop antibodies against the Rh protein. For a pregnant person who is Rh-negative carrying an Rh-positive baby, those antibodies can cross the placenta and attack the baby’s blood cells in subsequent pregnancies.
How Blood Type Is Inherited
You inherit one ABO gene from each biological parent. The A and B genes are co-dominant, meaning if you get one of each, both express themselves and you end up with type AB. The O gene is recessive, so it only shows up when you inherit it from both parents.
If you’re type A, you either got an A gene from both parents or an A from one and an O from the other. The same logic applies to type B. A person with type AB received an A gene from one parent and a B gene from the other. And to be type O, you need two copies of the O gene, one from each parent. This is why two type O parents will always have type O children, while two type A parents could potentially have a type O child if both carry a hidden O gene.
How Common Each Type Is
Blood type distribution varies by ethnicity and geography, but across populations, O is the most common group and AB is the rarest. Data from NHS Blood and Transplant in the UK provides a useful snapshot when positive and negative subtypes are combined:
- Type O: about 50% of donors (36% O+, 14% O-)
- Type A: about 36% of donors (28% A+, 8% A-)
- Type B: about 11% of donors (8% B+, 3% B-)
- Type AB: about 3% of donors (2% AB+, 1% AB-)
These proportions shift in different populations. Type B is more prevalent in parts of South and Central Asia, while type O dominates in Central and South America. Type A tends to be more common in European and Australian populations.
Who Can Donate to Whom
Blood type compatibility follows directly from the antigen-antibody system. If you receive blood carrying markers your body doesn’t recognize, your immune system attacks those cells. The rules break down like this:
- Type O- can donate red blood cells to anyone. Because O-negative blood has no A, B, or Rh markers, no recipient’s immune system will flag it as foreign. This makes O- the universal red blood cell donor.
- Type AB+ can receive red blood cells from anyone. Because AB-positive blood already carries every major marker, the immune system doesn’t see any incoming blood as a threat. This makes AB+ the universal recipient.
- Type A+ can receive from A+, A-, O+, and O-.
- Type B+ can receive from B+, B-, O+, and O-.
- Type O+ can only receive from O+ and O-.
Negative blood types are more restrictive. A- can only receive from A- and O-. B- can only receive from B- and O-. O- can only receive from other O- donors, making it both the most universally useful and the most limited for those who have it.
In emergency rooms, when a patient is bleeding critically and there’s no time to test their blood type, O-negative red blood cells are the default. Trauma guidelines recommend reserving O-negative supply specifically for women of childbearing age to protect against Rh sensitization, while O-positive may be used for other patients when O-negative stock is limited.
Beyond the Big Four
The ABO system is the most clinically important classification, but it isn’t the only one. Scientists have identified over 40 blood group systems involving hundreds of different surface markers. Most of these rarely cause problems in transfusions, but a few are significant in specific medical situations.
One striking example is the Bombay phenotype, sometimes written as Oh. People with this extremely rare blood type lack a foundational molecule called the H antigen, which serves as the building block for both A and B markers. Without it, their red blood cells can’t display A, B, or even the basic H structure. Standard blood typing reads them as type O, but they’re fundamentally different: they produce antibodies against A, B, and H antigens, meaning they can only receive blood from other Bombay-type individuals. The condition results from inheriting two nonfunctional copies of the gene responsible for building H antigens. It occurs in roughly 1 in 10,000 people in parts of India and is far rarer elsewhere.
For everyday purposes, though, the four ABO types plus the Rh factor cover the vast majority of what matters for transfusions, organ donation, and pregnancy planning. Knowing your type is straightforward: a simple blood draw and lab test can identify it in minutes.